1. Field of the Invention
The present invention relates generally to a corrosion resistant austenitic stainless steel. More particularly, the present invention relates to a stainless steel that is particularly well suited for use in an acidic environment. And still more particularly, the present invention relates to an austenitic stainless steel that is corrosion resistant in an environment of concentrated sulfuric acid.
2. Description of the Prior Art
It is known that stainless steels and alloys having high silicon content generally resist corrosion well in an environment of concentrated sulfuric acid, that is, concentrations of 90% or greater of sulfuric acid. As such, high silicon stainless steels and alloys are well suited for use as the materials of construction for equipment to manufacture concentrated sulfuric acid.
In applications such as sulfuric acid manufacturing facilities, corrosion resistance is not the only consideration that determines material selection. Cost considerations are always an important factor for material selection in any industry. Cost is particularly important in capital-intensive industries such as sulfuric acid manufacturing, due to the presence of low cost alternatives. One such alternative is brick-lined carbon steel. Brick-lined carbon steel is often selected over high silicon stainless steels and alloys because of its relatively low initial cost.
When silicon is added to an alloy, other alloying elements must be chosen so that the resulting alloy has acceptable corrosion resistance and other performance properties. For example, higher silicon austenitic alloys require additions of austenitizing elements, primarily nickel, for phase stability and ductility considerations. Higher alloy content translates to higher cost materials to the end user.
Based upon the above, the industry has commonly believed that in order to obtain superior corrosion resistance quantities in a concentrated sulfuric acid environment, an alloy must contain high amounts of silicon and/or other alloy elements. Conversely, alloys containing lesser amounts of alloying elements could be fabricated, but these alloys would not perform as well in corrosion resistance in certain concentrated sulfuric acid environments. Also, depending on the composition chosen, alloys with high amounts of alloying elements may be difficult to manufacture and fabricate. In discussing alloys herein, all compositions are provided by weight percentage.
The alloys that are commercially available and that are described in the patent literature demonstrate these common industry beliefs. For example, there have been commercial uses of high silicon alloys, such as 14.5% silicon cast iron having a designation of UNS F47003. However, these alloys are brittle and difficult to machine and weld, so that their manufacture is generally limited to castings. Thus, such material is not as versatile as wrought material in allowing a wide range of products.
A wrought alloy commonly used for its resistance to corrosion in a concentrated sulfuric acid environment is an alloy having the Unified Numbering System (UNS) designation of S30601. The UNS S30601 alloy has a composition of 0.015% max carbon, 17.0 to 18.0% chromium, 0.35% max copper, 1.0% max manganese, 0.05% max nitrogen, 17.0 to 18.0% nickel, 0.030% max phosphorous, 0.013% max sulfur, and 5.00 to 5.60% silicon. An alloy falling within the compositional ranges of UNS S30601 is disclosed in U.S. Pat. No. 4,543,244, U.S. Pat. No. 5,028,396 and U.S. Pat. No. 5,032,373, all to Jones et al.
Another alloy commonly used for applications demanding resistance to corrosion in a concentrated sulfuric acid environment is an alloy having the designation UNS S32615. The UNS S32615 alloy has a composition of 0.07% max carbon, 16.0 to 21.0% chromium, 1.5 to 2.5% copper, 2.0% max manganese, 0.3 to 1.5% molybdenum, 17.5 to 22.5% nickel, 0.045% max phosphorous, 0.030% max sulfur, and 4.8 to 6.0% silicon.
The UNS S30601 alloy has a relatively low alloy content but generally does not perform as well with respect to corrosion resistance as compared to other popular high silicon alloys, including the UNS S32615 alloy. Reportedly, the UNS S30601 alloy is not as resistant to process stream upsets (e.g., temperature or acid concentration excursions) as is that of the UNS S32615 alloy.
On the other hand, the UNS S32615 alloy performs relatively well in corrosion resistance. The higher nickel content of the UNS S32615 alloy coupled with copper and molybdenum aids the resistance to process upsets. However, this alloy has a relatively high alloy content. As a result, the UNS S32615 alloy would be relatively more costly to produce than the UNS S30601 alloy.
Other examples of high silicon alloys used for resistance to corrosion in a concentrated sulfuric acid environment include an alloy having a Werkstoff designation of 1.4390, which is marketed by VDM under the name 700Si Nicrofer.RTM. 2509Si7, as well as an alloy disclosed in Great Britain Patent No. 1,534,926. The Werkstoff 1.4390 alloy has a composition of 0.02% max carbon, 8.0 to 11.0% chromium, 2.0% max manganese, 0.50% max molybdenum, 22.0 to 25.0% nickel, and 6.5 to 8.0% silicon.
The Werkstoff No. 1.4390 alloy also performs relatively well with respect to corrosion resistance but it also has a relatively high alloy content. Furthermore, the high silicon content of the Werkstoff No. 1.4390 alloy also creates concerns for weld ductility, weld toughness and product toughness.
Finally, U.K. Patent No. 1,534,926 describes an alloy that exhibits good corrosion resistance in concentrated sulfuric acid environments. The U.K. Patent No. 1,534,926 describes at Table 3 several alloy composition variations of that invention, which alloy compositions are numbered 2 through 9. As can be seen in those described alloy compositions, either the amount of silicon is relatively high (i.e., above 6.5%) and/or the amounts of the other alloying elements are relatively high. For the alloy compositions having higher amounts of silicon, workability would suffer. And for those alloy compositions for which the sum of all alloying elements are higher, those alloys will be relatively costly to produce.
Therefore, there is a need in the stainless steel and alloy industry to create a material with a relatively low content of alloying elements, and thus being relatively inexpensive to produce while maintaining a relatively high level of corrosion resistance in a concentrated sulfuric acid environment. Accordingly, it is an object of the present invention to provide austenitic stainless steel that will be corrosion resistant in a concentrated sulfuric acid environment while optimizing the various alloying elements to provide products that can be efficiently wrought from a low cost combination of alloying elements.